This invention relates to a process for generation of acid, and to a medium for use in this process. Preferred forms of the present process are useful for generating images. This invention also relates to trisubstituted pyridine indicator dyes which are useful in forming images by this and other acid-mediated processes (the term acid-mediated is used herein to refer to a process in which a radiation-dependent process is used to form an acid, and this acid serves to change the color of an indicator dye).
Images can be generated by exposing a photosensitive medium to light in an imagewise fashion. Some conventional non-silver halide photosensitive compositions contain molecules which are inherently photosensitive, so that absorption of electromagnetic radiation brings about decomposition oft at most, as many molecules as photons absorbed. However, a dramatic increase in the sensitivity of such photosensitive compositions can be achieved if the absorption of each photon generates a catalyst for a secondary reaction which is not radiation-dependent and which effects conversion of a plurality of molecules for each photon absorbed. For example, systems are known in which the primary photochemical reaction produces an acid (which will hereinafter be called the "primary acid" or "first acid"), and this acid is employed catalytically to eliminate acid-labile groups in a secondary, radiation-independent reaction. Such systems may be used as photoresists: see, for example, U.S. Pat. Nos. 3,932,514 and 3,915,706; and Ito et al., "Chemical Amplification in the Design of Dry Developing Resist Materials", Polym. Sci. Eng., 23(18), 1012 (1983).
Among the known acid-generating materials for use in this type of process employing secondary, non-radiation dependent reactions are certain diazonium, phosphonium, sulfonium and iodonium salts. These salts, hereinafter referred to as superacid precursors, decompose to produce superacids, i.e., acids with a pKa less than about 0, upon exposure to electromagnetic radiation. Other materials decompose to produce superacids in a similar manner. However, in the absence of a spectral sensitizer, the known superacid precursors decompose to produce superacid only upon exposure to wavelengths which the precursors absorb, which are typically in the short ultraviolet region (below about 280 nm). The use of such wavelengths is often inconvenient, not least because special optical systems must be used.
It is known that various dyes can sensitize the decomposition of superacid precursors upon absorption by the dye of radiation which is not significantly absorbed by the superacid precursor; see, for example, European Patent Application Publication No. 120,601. Unfortunately, however, due to the very low pKa of the superacid, many such dyes are protonated by the superacid, so that no unbuffered superacid is produced (i.e., the sensitizing dye buffers any superacid produced). Since no unbuffered superacid is released into the medium, decomposition of superacid precursors sensitized by these dyes cannot be used to trigger any secondary reaction which requires the presence of unbuffered superacid.
(The term "unbuffered superacid" is used herein to refer to superacid which is not buffered by the sensitizing dye, and which thus provides an acidic species stronger than that provided by the protonated sensitizing dye. Because of the extreme acidity of superacids and their consequent tendency to protonate even species which are not normally regarded as basic, it is possible, and indeed likely, that "unbuffered superacid" will in fact be present as a species buffered by some component of the imaging medium less basic than the sensitizing dye. However, such buffering by other species may be ignored for the present purposes, so long as superacid is present as an acidic species stronger than that provided by superacid buffered by the sensitizing dye.) Crivello and Lam, "Dye-Sensitized Photoinitiated Cationic Polymerization", J. Polymer Sci., 16, 2441 (1978) and Ohe and Ichimura, "Positive-Working Photoresists Sensitive to Visible Light III, Poly(tetrahydropyranyl methacrylates) Activated by Dye-Sensitized Decomposition of Diphenyliodonium Salt", J. Imag. Sci., Technol., 37(3), 250 (1993) describe small sub-groups of sensitizing dyes which are sufficiently non-basic that the buffered superacids produced can effect certain acid-catalyzed reactions. However, the need to restrict the choice both of sensitizers and of acid-catalyzed reactions may make it difficult to design an efficient imaging system at a specific desired wavelength.
A variety of non-basic, polycyclic aromatic compounds sensitize decomposition of superacid precursors to produce unbuffered superacid upon exposure to longer wavelengths than the superacid precursors absorb themselves. Such materials are discussed in, for example, DeVoe et al., "Electron Transfer Sensitized Photolysis of 'Onium salts", Can. J. Chem., 66, 319 (1988); Saeva, U.S. Pat. No. 5,055,376; and Wallraff et al., "A Chemically Amplified Photoresist for Visible Laser Imaging", J. Imag. Sci. Technol., 36(5), 468-476 (1992).
U.S. Pat. Nos. 5,286,612 and 5,453,345 describe a process by which a wider variety of dyes than those discussed above may be used together with a superacid precursor to generate free (unbuffered) superacid in a medium. In this process, acid is generated by exposing a mixture of a superacid precursor and a dye to actinic radiation of a first wavelength which does not, in the absence of the dye, cause decomposition of the superacid precursor to form the corresponding superacid, thereby causing absorption of the actinic radiation and decomposition of part of the superacid precursor, with formation of a protonated product derived from the dye; then irradiating the mixture with actinic radiation of a second wavelength, thereby causing decomposition of part of the remaining superacid precursor, with formation of unbuffered superacid. Generation of superacid by exposure to the second wavelength may be sensitized by one of the non-basic, polycyclic aromatic sensitizers mentioned above. (For convenience, the type of process disclosed in this patent will hereinafter be called the '612 process.)
U.S. Pat. Nos. 5,334,489 and 5,395,736 describe processes for the photochemical generation of acid and for imaging using conventional ultra-violet sensitizers; these processes will hereinafter collectively be called the '489 process.
U.S. Pat. No. 5,441,850 and its continuation-in-part, copending application Ser. No. 08/430,420, filed Apr. 29, 1995 (and the corresponding International Application No. PCT/US95/05130, Publication No. WO 95/29068) all describe a variation of the aforementioned '612 process which uses an imaging medium comprising a sensitizing dye having a first form and a second form, the first form having substantially greater absorption in a first wavelength range than the second form. The medium is exposed to actinic radiation in this first wavelength range while at least part of the sensitizing dye is in its first form so that the sensitizing dye decomposes at least part of a superacid precursor, with formation of unbuffered superacid. The medium is then heated to cause, in the exposed areas, acid-catalyzed thermal decomposition of a secondary acid generator and formation of a secondary acid. This secondary acid brings about a change in absorption of an image dye and thereby forms an image. Finally, in the non-exposed areas of the medium, the sensitizing dye is converted to its second form, thus removing the absorption in the first wavelength range caused by the first form of the sensitizing dye, and lowering the minimum optical density (D.sub.min) in this wavelength range. (For convenience, the type of process disclosed in this patent and these applications will hereinafter be called the '850 process.)
The entire disclosures of the aforementioned U.S. Pat. Nos. 5,286,612; 5,453,345; 5,334,489; 5,395,736 and 5,441,850 and copending application Ser. No. 08/430,420 are herein incorporated by reference.
The aforementioned '612, '489 and '850 processes all make use of a secondary acid generator, the thermal decomposition of which is catalyzed by the unbuffered superacid produced in the primary, radiation-dependent reaction. In effect, the secondary acid generator acts as an "acid amplifier" which causes the generation of multiple moles of the secondary acid from each mole of unbuffered superacid produced in the primary reaction, and thus increases the sensitivity of the medium, as compared with a medium relying only upon the generation of unbuffered superacid. The specific preferred secondary acid generators described in these processes are esters of squaric and oxalic acids. In these esters, a basic site is protonated by the first acid, and thereafter a leaving group is released from this first site, leaving an acidic proton at the site. For example, in the squaric acid diester of the formula: ##STR1## protonation occurs at one of the oxygen atoms, ultimately resulting in the formation of a hydroxyl group attached to the four-membered ring (the proton of this hydroxyl group is of course strongly acidic in squaric acid derivatives).
A secondary acid generator should have high acid sensitivity (i.e., it should readily undergo thermal decomposition in the presence of the first acid), but should also have high thermal stability in the absence of this acid. In the aforementioned squaric and oxalic acid ester secondary acid generators, because the secondary acid-generating reaction involves only a single site, it is difficult to improve the acid sensitivity of the secondary acid generator by chemical modifications without adversely affecting its thermal stability, and vice versa.
Moreover, in the aforementioned squaric and oxalic acid ester secondary acid generators, the secondary acid released is incapable of protonating the secondary acid generator (or, in more strictly accurate thermodynamic terms, the equilibrium proportion of secondary acid generator protonated by the secondary acid is so low as to have a negligible effect on the decomposition of the secondary acid generator). If such protonation of the secondary acid generator by the secondary acid could be made to occur, the thermal decomposition of the secondary acid generator would also be catalyzed by the secondary acid, and thus this thermal decomposition would be autocatalytic. Such autocatalytic thermal decomposition is desirable in practice because the number of moles of secondary acid which can be generated directly from a single mole of unbuffered superacid is limited (presumably by factors such as, for example, the limited rate of diffusion of secondary acid generator through the polymeric binders which are usually used in imaging media of the aforementioned types) and autocatalytic thermal decomposition can increase the number of moles of secondary acid generated from a single mole of superacid, and thus increase the sensitivity of the imaging medium.
Secondary acid generators have now been developed in which the secondary acid-forming reaction involves two separate sites within the molecule; such secondary acid generators can comprise a first site having a relatively basic "trigger" group which is first protonated by the superacid with a second site bearing a leaving group which forms a strong secondary acid. Some of the secondary acid generators developed by applicants undergo autocatalytic thermal decomposition.
Also, a persistent problem in acid-mediated color imaging processes is finding a yellow indicator dye with sufficient photostability. Trisubstituted pyridine dyes bearing at least one N,N-dialkylaminophenyl substituent are among the most satisfactory yellow image dyes available commercially of which the neutral form has sufficient basicity for use in acid-mediated processes. However, the protonated forms of such dyes, which are present in regions of the final image having yellow density, are susceptible to photobleaching, for example when an image in the form of a slide is projected and hence subject to prolonged exposure to a large radiation flux.
Moreover, in the '850 process, as described above, a sensitizing dye is required having a first form and a second form, the first form having substantially greater absorbance in a first radiation range than the second form. In the case of a sensitizer to blue light, the first form of the sensitizing dye (which absorbs blue light) is a yellow dye. After exposure, the film is heated, thus converting the sensitizing dye to its second (colorless) form in the non-exposed areas. In exposed areas, however, the sensitizing dye may remain in its first form. Accordingly, in the '850 process, at least part of the final yellow image may comprise the first form of the sensitizing dye. Therefore, a sensitizing dye for use in the '850 process must, in its first form, not only function as an photosensitizer for the superacid precursor, and be less basic than the secondary acid generator (properties which are required for the efficient photogeneration and amplification of acid), but must also be sufficiently light-fast to be used to form part of the final image. Prior art pyridine indicator dyes in which one substituent is an aryl group bearing an N-aryl-N-alkyl grouping, although more light-fast than the analogous, commercially-available dyes bearing an N,N-dialkylaminophenyl substituent mentioned above, are too basic in their monoprotonated forms to be used as sensitizer dyes in the aforementioned '850 process with typical secondary acid generators.
It has now been found that the photostability of trisubstituted pyridine indicator dyes is acceptable if at least one of the substituents is an aryl group bearing an N,N-diarylamino grouping. Furthermore, the protonated forms of the same N,N-diarylamino substituted dyes are useful in the aforementioned '612, '489 and '850 processes for sensitizing superacid precursors to blue visible or similar radiation; in the '850 process, the protonated form of the same dye can be used both as sensitizer and image dye.